1. Field of the Invention
The present invention relates to a powder detergent that incorporates post-added acidulant and whitening agent particles and a method of making such a detergent. In particular, the powder contains a high level of surfactant yet is free flowing, does not cake, dissolves in cool water and does not suffer from bulk color deterioration during storage. The powder detergent contains discrete whitening agent particles to alleviate the bulk color deterioration typically associated with the degradation of fluorescent whiteners in nonionic containing powder detergents. The powder detergent also includes an acidulant to improve the solubility in the laundering solution as well as to improve the dissolution of the powder detergent in those washing machines having a detergent dispenser.
2. Discussion of Related Art
There is an on-going effort to provide powdered laundry detergents having an increased amount of detergent surfactants. The benefits of highly concentrated detergents include a savings in packaging use and cost. Unfortunately, there are limits to the amount of detergent surfactant that can be included in a powdered detergent while still providing the consumer desired characteristics of flowability, solubility, cleaning and whitening performance.
Most granular detergents are produced by spray drying. This process involves mixing detergent components such as surfactants and builders with water to form a slurry which is then sprayed into a high temperature air stream to evaporate excess water and to form bead-type hollow particles. While spray drying the detergent slurry produces a hollow granular detergent having an excellent solubility, extremely large amounts of heat energy are needed to remove the large amounts of water present in the slurry. Another disadvantage of the spray drying process is that because large scale production equipment is required, a large initial investment is necessary. Further, because the granules obtained by spray drying have a low bulk density, the granule packaging volume is large which increases costs and paper waste. Also, the flowability and appearance of the granules obtained by spray drying may be poor because of the presence of large irregularities on the surface of the granules.
In addition to these characteristic processing and product problems associated with the spray drying process, volatile materials, such as nonionic surfactants, are emitted into the air when processed by this method. This volatilization problem, manifested by the discharge of dense "blue" smoke from the spray tower, is referred to as "pluming." Air pollution standards limit the opacity of the plume. Consequently, it is necessary to limit the capacity of the spray tower or, in extreme instances, discontinue operation.
In an attempt to avoid the problems caused by spray drying, considerable developmental effort has focused on post-dosing the product with nonionic surfactants after the spray drying operation. Unfortunately, post-dosing of the spray dried base with surfactant in amounts sufficient to provide satisfactory wash performance generally results in a product that has poor dissolution characteristics. Accordingly, the amount of surfactant that may be employed in the detergent formulation is severely limited. Because heavy-duty laundry detergents need large amounts of nonionic surfactant, inorganic silicas have been added to these detergent formulations to absorb the nonionic liquids.
For example, U.S. Pat. No. 3,769,222 to Yurko et al. discloses mixing liquid nonionic surfactants with sodium carbonate until partial solidification occurs followed by the addition of large amounts of silica (silicon dioxide) to produce a dry free-flowing detergent composition. A disadvantage to this technique, however, is that because the silica has no significant cleaning activity, its inclusion in a detergent formulation in large amounts merely serves to increase the cost of the product. Further, the use of silica in detergents adds to the total suspended solids (TSS) content of laundry waste water contrary to the dictates of many local and state water pollution standards. Therefore, there is an incentive to keep low the amount of silica added to the detergent composition.
U.S. Pat. No. 4,473,485 to Greene reports that a free-flowing granular detergent can be prepared by mixing a polycarboxylic structuring agent solution with a micronized carbonate followed by the addition to the mixture of a nonionic surfactant and water, followed by removal of the excess water. The preferred micronized carbonate is calcium or sodium carbonate. A disadvantage of this process, however, is that the micronized carbonate used by Greene to enhance the flowability of the detergent product is quite expensive as compared to standard sodium carbonate. Without the use of the micronized carbonate, Greene's product would not have such good flowability. In addition, where the micronized carbonate is calcium carbonate, the building capability of the detergent is reduced.
Therefore, a need exists for a process that substantially overcomes the problem of free-flowability in highly loaded detergents, particularly highly loaded nonionic detergents. At the same time, these highly loaded, high density, powder detergents must dissolve under cool and/or cold water conditions that are becoming more prevalent world-wide. Granular laundry detergents containing admixed sodium carbonate are known to exhibit poor solubility under certain conditions. This poor solubility can cause clumps of detergent, which appear as solid white masses remaining in the washing machine and on washed clothes. Such clumps usually occur when the detergent is placed in a pile in the washing machine, particularly during cold water washes and/or when the order of addition to the washing machine is laundry detergent first, clothes second, and water last. The clumps may also occur when the powdered detergent is trapped within the folds or pockets of the fabrics to-be-washed, particularly in machines that do not provide for adequate agitation. It is believed that one contributor to this solubility problem is caused by hydration of the sodium carbonate and/or particle bridging resulting in a poorly soluble mass before the granular detergent can be dispersed and solubilized in the laundering solution.
Another problem exists when the laundry detergent contains high levels of nonionic surfactant. When such a detergent is added to the wash water, particularly when the temperature of the wash water is cool, the nonionic surfactant does not immediately solubilize. Instead, the surfactant may tend to gel resulting in a sticky mass which may deposit on the fabric before sufficient wash water is present to solubilize the nonionic surfactant.
U.S. Pat. No. 5,300,250 to Morgan et al. discloses that the addition of low levels of hydrophobic amorphous silicate material to granular laundry detergents containing admixed sodium carbonate improves their solubility in the laundering solution and eliminates or reduces the problem of clumps remaining in the washing machine and on washed clothes. The hydrophobic amorphous silicate material acts as an anti-caking agent and flow aid. The detergent is prepared by spray drying aqueous crutcher mixes of the surfactant and additives together with a premix containing sodium carbonate and hydrophobic amorphous silicate material.
U.S. Pat. No. 5,338,476 to Pancheri et al. discloses that spray dried granular laundry detergents having admixed sodium carbonate can achieve improved solubility in the laundering solution by incorporating citric acid. They report that they believe that the citric acid rapidly reacts with the sodium carbonate in the laundering solution to release carbon dioxide and helps to disperse the detergent and minimize the formation of insoluble clumps. The use of citric acid, in this manner, however, may not be desirable because a substantial portion of the citric acid may become neutralized to sodium citrate during storage. It is believed that the citric acid, which is hydroscopic, will absorb the free water present in the powder detergent formulation as well as in the atmosphere and become neutralized. The neutralization causes an unwanted increase in detergent particle size, powder lumps in the box, and loss of the desired effervescent effect.
U.S. Pat. No. 5,002,758 to Ichii et al. discloses bubbling bathing preparations preferably in the form of a tablet that contain fumaric acid and a carbonate together with carboxymethyl cellulose or an alkali metal salt or polyethylene glycol and less than 0.1% of a nonionic surface active agent. They also disclose that other organic acids may be used, for example, citric, tartaric, malic, malonic, pyridone carboxylic, succinic, adipic, phosphoric, and their salts.
A particular problem arises with the use of high density laundry detergent powders, i.e., those with bulk densities of 650 g/l or greater. Denser powders such as those of 800 g/l or higher are even more problematic. While these powders provide consumers the benefit of concentration and lower dosages, the processes required to produce high densities leave little or no void space in the detergent powder. For example, U.S. Pat. No. 5,133,924 describes a process that reduces the intraparticle porosity so that the void space is substantially decreased. These highly concentrated powders, however, can prove difficult to dissolve since the powder has little or no free space to allow the entry of water necessary for dissolution. This, in turn, can result in the powder forming localized areas of gelation which remain undissolved at the end of the wash cycle and contribute to residue. As a result, they are more susceptible to the cold water clumping problems.
U.S. Pat. No. 5,415,806 to Pepe et al. describes high density laundry detergent compositions having a bulk density of 650 g/l or greater and intraparticle porosities of about 25% or less. They report that acceptable solubility and dispersion is achieved by including a C.sub.2-4 alkylene oxide condensation product as a solubility aid. The process of making the described detergent composition includes preparing a base powder by mixing water plus detergent components in a slurry and spray drying the slurry. Consequently, the described process does not offer an improvement to the known disadvantages of spray drying. In addition, the compositions are those with high density but low porosity. As a result, the amount of surfactant that can be effectively loaded is restricted. Moreover, without the solubility aid it is likely that the detergent would not be effectively dissolved or dispersed.
Another problem that exists with the use of higher density powder detergents is that they are not completely dispensed when used in automatic washing machines or those types of washing machines that are prevalent in Europe. In those machines, water enters the dispenser which is charged with the powder detergent and flushes the powder into the wash liquor. If the water does not flush out the entire amount of powder, the powder, when it solidifies, can form relatively large clumps that can eventually block the dispenser and/or feed pipe from the dispenser to the washing compartment of the machine. This wastes detergent and produces a lower level of cleaning. It also requires the user to clean out the dispenser and/or feed line, preferably after each wash cycle.
This problem is more prevalent with higher density powders, particularly in non-phosphate, zeolite-containing products and at low wash temperatures, including cold-water washes, and at low water pressure and/or water flow rates. While the phenomenon is not fully understood, solubilization of at least a portion of the powder detergent to form a pasty or syrupy consistency slurry before the powder has been washed out of the dispenser into the wash liquor appears to be a contributing factor.
Therefore, there is also a need for a powder detergent that will effectively dissolve in cool water, particularly those powdered detergents that contain high levels of surfactants. One problem with powder detergents containing high levels of nonionic surfactants, however, is that they may detrimentally affect whitening agents added to the detergent.
For example, it is known to add whitening agents to washing detergents in order to enhance the whiteness and brightness of the washed textiles. In particular, fluorescent whitening agents (FWAs) counteract the yellowing of cotton and synthetic fibers. FWAs are adsorbed on fabrics during the washing process. FWAs function by absorbing ultraviolet light, which is then emitted as visible light, generally in the blue wavelength ranges. The resultant light emission yields a brightening an whitening effect, which counteracts yellowing or dulling of the fabric. If, however, the whitener, particularly a fluorescent whitener, is incorporated in powdered detergents in the customary manner, they have an exceedingly undesirable drawback. Frequently, they cause a deterioration in the bulk appearance of the detergent. Unattractive, yellow or greenish-yellow powders of reduced commercial value are produced. Without being bound by any particular theory, it is believed that the whitening agents react with the detergent surfactants causing the agent to change forms and thereby cause the bulk appearance of the detergent to change. This reaction appears to be particularly prevalent when the detergent contains a substantial amount of nonionic surfactant.
One solution that has been proposed is to select a fluorescent whitening agent that may be more stable in a detergent containing a high nonionic surfactant concentration. The drawback to such whitening agents is that they lack cold water performance and they are expensive.
Another solution that has been proposed is reported in U.S. Pat. Nos. 4,298,490 and 4,309,316 to Lange et al. In these patents, a fluorescent whitener such as a bis-styrylbiphenyl, a bis-triazoylstilbene or naphthotriazolylstilbene type, is dissolved or dispersed in a mixture of water and a polymer (polyvinyl alcohol or polyvinyl pyrrolidone) and then added to the detergent slurry which is then later dried. Alternatively, the whitener solution or dispersion may be spray dried, suspended in water, added to the detergent slurry and then spray dried. These methods, however, require many processing steps prior to incorporation into a detergent slurry.
Therefore, there is a need for a powder detergent that contains a high level of surfactant to achieve desirable cleaning performance, yet is able to dissolve in cold water conditions and provide a whitening agent so that the bulk powder detergent does not suffer from discoloration upon storage.